In recent years, there is a dramatic trend in telecommunications to use packet-switched networks over circuit-switched networks for voice communications. Packet-switched networks offer more versatility, flexibility, and media capabilities than circuit-switched networks and at a lower cost. Although the quality of voice communications is generally not as high as circuit-switched networks, dramatic gains have been made in the Quality of Service or QoS of packet-switched voice communications.
Compared to circuit-switched networks, many IP-PBX systems, however, continue to suffer from reliability issues. Many Internet Protocol-Private Branch Exchange or IP-PBX systems are susceptible to serious/severe outage when the IP network is disrupted. Disruptions can occur, for example, due not only to contact with a prolific virus or worm but also the activities of hackers. When a disruption occurs, the engineering complexities of the platforms supporting the Local Area Network IP telephony can cause extreme difficulties in identifying the source of the disruption and lengthen the period of the disruption relative to circuit-switched networks. Moreover since servers for Voice over IP or VoIP calls answer incoming calls automatically, incoming calls to a terminal experiencing a disruption are typically lost, which can cause caller frustration.
Because the system is IP-centric, fallback mechanisms are often limited and often undesirable. While technology is available to re-route packet-switched or circuit-switched calls over different facilities when primary facilities are unavailable, many customers do not want a backup analog or DCP station sitting on their desk as a backup in the event of IP phone failure. Technology is also available, such Avaya Inc.'s EC500™ product, to bridge an incoming call to a desk phone to a cellular phone. This alternative has the drawback of having two phones ringing in the user's office at the same time even though no problem with the IP telephony is encountered the vast majority of the time. Finally, some enterprise networks have used a parallel network strictly for voice communications. While a parallel network can provide the necessary protection from the problems with general data traffic, it is an expensive solution.